Excellent agreement in the percent of cells in the S phase as determined by the labeling index and DNA histogram using the cytofluorograf 4802A was found in a majority of samples of marrow from children with acute leukemia. However, in a minority of cases there were gross differences. Interpretation of DNA histograms have been compared using the Fried method and probability paper. Thus far our data seems to be better interpreted using probability paper. The in vivo effect of dianhydrogalactitol (DAG) and VM26 singly on the cell cycle has been evaluated in 10 children (6 with acute leukemia and 4 with advanced solid tumors). Inhibition of DNA synthesis occurred in both normal and leukemic cells after DAG and a G2 block was also seen in two leukemic patients. In each case there was a block in G2 after VM26. The Robinson and Pike and Dicke, Spitzer et al methods for evaluating marrow proliferation were evaluated on samples of cord blood, normal bone marrow and leukemic marrow (ALL, AML and CML). Our data confirms the results previously found using these methods of culture of marrow. Both methods were helpful in following the response to therapy of 5 children followed serially with CML. Interestingly, blasts grew also from cord blood. Lastly, leukemic cells from a patients with acute undifferentiated leukemia have been growing in continuous culture for one year. These cells are EB virus negative and recently have grown in nude mice.